Colorectal cancer (CRC) is a common disease but disproportionately adversely affects African Americans over other racial groups, with younger age of presentation, a more advanced stage, and higher mortality. The reason for this adverse outcome is not clear. We show in preliminary data that CRCs from African Americans from a population-based cohort have a lower prevalence of microsatellite instability (MSI, caused by major DNA mismatch repair [MMR] deficiency), a biomarker associated with better survival presumably due to surrounding immune cells, but have a higher prevalence of elevated microsatellite alterations at selected tetranucleotide repeats (EMAST), a biomarker we show associated with adenoma-to-carcinoma progression, advanced staged cancers, and higher degrees of tumor immune cell infiltration. EMAST appears to be an acquired defect in colorectal tumors that may be a result of tumor inflammation, and is associated with heterogeneous loss of expression of the minor MMR protein hMSH3. We hypothesize that the immune cell profiles from MSI tumors are different than EMAST tumors (low and high prevalence among African Americans, respectively). In this proposal, we will explore racial differences in immune profiles within colorectal cancers, including survival prognostication, utilizing samples and data from the North Carolina Colon Cancer Study, the North Carolina Rectal Cancer Study, the Colon Cancer Family Registry, as well as other specimens. We will examine the immune profiles of EMAST and MSI tumors to help determine the type of immune cells associated with each biomarker. We will also compare immune profiles between race and genomic instability to further correlate differences. The information obtained from work in this proposal may explain some of the racial differences observed in colorectal cancer, and direct future investigation on differences between acquired minor and major MMR defects on activating the immune system.